Semiconductor element, electronic device

ABSTRACT

A current storing circuit capable of having a small area, a simple structure with the small number of devices, a low consumption current operation and high yield in manufacturing is provided. Applying the current storing circuit to the current-driving type of display device such as an OLED display device can improve the aperture rate of pixels and reliability of the display device as well as highly functionalize the display device. The invention is characterized by using a new semiconductor element in a shape of a transistor having plural drains or sources. When the semiconductor elements is used for both of a writing element and a driving element, reading in and storing a current value and outputting the current can be performed by only the two semiconductor elements, so that the area occupied by the devices would be easily reduced significantly.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a configuration of a semiconductorelement including a field-effect transistor type and an electric circuitusing the semiconductor element. Further, the present invention relatesto a light emitting device wherein a light emitting element and thesemiconductor element controlling the light emitting element areprovided on the light emitting device. Or, the present invention relatesto a display device. Moreover, the present invention relates to anelectronic apparatus on which the light emitting device and the displaydevice are mounted.

2. Description of the Related Art

In recent years, the importance of a light emitting device displayingimages is increased. As the display device, a liquid crystal displaydevice displaying images by using a liquid crystal element is widelyused as a display device for various kinds of uses including a cellularphone, a personal computer and the like by utilizing the advantages ofthe liquid crystal display device such as high picture quality, thinnessand lightweight.

On the other hands, the development of a light emitting device and alight emitting display device using a light emitting element whichserves as a self-luminous element has also been advanced. Suchself-luminous element includes various kinds of elements in the widelyrange of an organic material, an inorganic material, a thin filmmaterial, a bulk material, and a dispersion material.

Especially, a representative self-luminous element is an organic lightemitting diode (OLED) element. An OLED display device using the OLEDelement as a light emitting element has, in addition to features thatthe OLED display device is thinner and lighter than the conventionalliquid crystal display devices, new features such as a first responsespeed, a wide viewing angle and a low voltage drive which are suitablefor an animation display. Therefore, the OLED display device is drawingattention as the next-generation display device since the OLED displaydevice is considered wide range of uses such as a cellular phone, aportable information terminal (PDA: personal digital assistant),television, and a monitor.

In particular, an active matrix (AM) type OLED display device canprovide a high definition display in a large size screen, which isdifficult to a passive matrix (PM) type OLED display device.Furthermore, the AM type OLED display device operates at lower powerconsumption than the PM type OLED display device, and has a highreliability. Thus, the AM type OLED display device is expected to beimplemented.

One of various conditions necessary for putting a light-emitting devicesuch as an OLED display device into practice is to maintain the luminousintensity almost constant. Especially an OLED device has a problem thatthe luminous intensity greatly depends on ambient temperature. In manyOLED devices, the amount of the current increases in high temperatureunder the condition that the voltage is constant. The greater the amountof the current flowing to the OLED device is, the higher the intensityof the OLED device becomes.

Then, the OLED light-emitting device would be unstable and very bad forusage such that the display of the whole screen changes in brightness inaccordance with the change of temperature.

There is also a problem that an existing OLED device generally tends tobe decreased, as the time elapses, in the luminous intensity due tolight emission. This is a fairly serious problem although the degree ofdecrease of the luminous intensity is broad depending on the structureof the OLED device.

When the luminous intensity decreases as the time elapses due to theamount of light emission and thereby the luminous intensity cannot bemaintained almost constant, the display of a light-emitting device isnot only unstable in the brightness as a whole but also has a problem indisplaying gradation in each pixel. For example, displaying inrespective pixels in a screen of a static image having significantdifference in the luminous intensity over a long time causes burning ofan image, which becomes so ugly in appearance.

Especially in the case of an OLED display device for displaying a colorimage by means of three kinds of luminous element corresponding to R(red), G (green) and B (blue), used generally “a triple color paintingmethod” in which an OLED device to be used is different by a color, fromthe view of high efficiency and low consumption current in emitting alight. Then, color shift occurs since the temperature dependency of theluminous intensity is different by a color. Otherwise, color shiftbetween the displaying colors in a light-emitting device occurs becausethe luminous intensity of respective OLED devices decrease at adifferent speed corresponding to every color as the time elapse.

The decrease of the luminous intensity of an OLED device in elapse oftime becomes larger in the case that the voltage applied to the OLEDdevice is constant (constant voltage driving) than the case that thecurrent flowing to the OLED device is constant (constantcurrent-driving). The reason is as follows.

It is said that the luminous intensity L of an OLED device is generallyproportional to the amount of the current flowing to the OLED device I(V). When the proportional constant is c (V), there is a relationexpressed by L=c(V)I(V), wherein V is the applied voltage to the OLEDdevice necessary for emitting a light at the intensity L.

Continuous emission of a light by an OLED device, however, graduallydecreases both of c (V) and I (V). Here, in the case of constantvoltage-driving of a OLED device, the decrease of the both of c (V) andI (V) is reflected to that of L. On the other hand, in the case ofconstant current-driving of an OLED device, the decrease of c (V) isonly reflected to that of L. Therefore, the decrease of L is larger incomparison in the constant voltage-driving than in the constantcurrent-driving.

As a background of the decrease of c (V), an OLED device originally haslittle resistance against moisture, oxygen, light and heat, and thereby,change of characteristics and deterioration of the device per se tend tostart or be facilitated easily. However, the progressing speed of thedeterioration of the device greatly depends on the kind of a luminousmaterial, the material of an electrode, the structure of a devicedriving the light-emitting device, the manufacturing circumstance andthe manufacturing conditions. Improving the above, therefore, canrestrain to a certain degree the decrease of c (V) in elapse of time.

Further, the temperature dependency of the luminous intensity of an OLEDdevice is significantly high in the constant voltage-driving whereasthat is often low in the constant current-driving. This can beconsidered that I (V) has high temperature dependency while c (V) haslittle temperature dependency under a condition of 1=c(V)I(V).

In view of the above, gradation display by current-driving rather thanthe voltage-driving of a luminous element of an OLED light-emittingdevice must enable the luminous element to maintain the almost constantintensity without greatly decreasing the luminous intensity in theelapse of time and without depending on the change of ambienttemperature.

In the case of luminous elements other than the OLED device, thetemperature dependency is also low in the constant current-drivingrather than in the constant voltage-driving, generally, although it alsodepends on the kind of the device. From this point of view, the constantcurrent-driving is still more preferable.

In a light-emitting device such as an AM-type of OLED display device,the current-driving of a luminous element is possible by mounting acurrent storing circuit on a pixel. The current storing circuit to bemounted on a pixel can be produced by means of an active element such asa thin film transistor (TFT).

Not only the current storing circuit but a circuit in a pixel isgenerally preferable to have a structure as simple as possible from aview of reduction of a manufacturing cost and lowering of the defectiverate.

Moreover, the area occupied by a circuit is preferably as small aspossible since improvement of the luminous area rate (the aperture rate)is strongly required in order to save power and stabilize emission of alight. The small luminous area rate, however, requires the luminouselement to emit a light at the high current density for the purpose ofobtaining the predetermined intensity, so that the luminous elementwould be easily facilitated to be changed in characteristics anddeteriorate.

The most direct and effective method for improving the luminous arearate (the aperture rate) is to mount a circuit in a pixel on the sideopposite to the direction of light-emission. This method, however, isnot an effective solution in practice since it is better to mount acircuit in a pixel on the same side as the direction of light-emissionin order to stably produce an OLED device.

Another reason why the small circuit area is preferable is that it ispossible to highly integrate a pixel circuit for high functionalization.

SUMMARY OF THE INVENTION

In view of the above, an object of the invention is to provide alight-emitting device with a simple structure in which a luminouselement can maintain an almost constant intensity without depending onambient temperature and without greatly decreasing the luminousintensity as the time elapses. Another object of the invention is toprovide alight-emitting device with a simple structure in which desiredcolor display is possible without any color shift. Yet another object ofthe invention is to provide a structure of a semiconductor elementavailable to put the above-mentioned light-emitting device into practiceand an electric circuit using the semiconductor element.

In the invention, a luminous element, a driving element for controllingthe luminous element and a writing element are first provided. Theelements other than the luminous element are generally formed by meansof a TFT, which is, however, disadvantageous since the number of TFTincreases and the circuit area including wiring thereby increases. Inthe invention, the circuit is simplified and reduced in the area byusing the following new device.

The above-mentioned new element is in a shape of a transistor having aplurality of drains and is referred to as a multi-drain transistor inthis specification. The multi-drain transistor is, in other words, asemiconductor element having a gate electrode and at least threeimpurity regions.

More particularly, it can be said that the multi-drain transistor is asemiconductor element, which has a semiconductor layer, a gateinsulating film formed so as to cover the semiconductor layer and a gateelectrode contacting with the gate insulating film, the semiconductorlayer having a channel forming region and at least three impurity dopedsource or drain regions, the channel forming region and the gateelectrode sandwiching the gate insulating film therebetween andoverlapping each other, and said at least three impurity regionscontacting with the channel forming region. It is assumed here that oneof the impurity regions is a source and the others are drains.

It should be noted that in some cases the multi-drain transistor isproperly referred to as a multi-source transistor or a multi-sourcemulti-drain transistor in accordance with the way of use. Generally, asource and a drain of a transistor (especially a TFT) are often same instructure and cannot be necessarily discriminated apparently. Theabove-mentioned device is therefore collectively called a multi-draintransistor here, including a multi-source transistor and a multi-sourcemulti-drain transistor.

The shape, including the size and symmetry, of a multi-drain transistoris not especially limited. A semiconductor used for producing amulti-drain transistor may be in any form such as a compositionmaterial, bulk, a non-crystal (amorphous) thin film and apolycrystalline (poly-) film.

It is most practical at present to use a thin film semiconductor made ofpolycrystalline silicon (poly-silicon) for a driving element forcontrolling a luminous element. The channel type, including with orwithout symmetry, of respective drains and sources of a multi-draintransistor is also not especially limited.

The invention further provides an electric circuit using a writingelement and a driving element. A multi-drain transistor is used in oneor both of the writing and driving elements. The invention having suchstructure contributes to simplification, reduction in the area and highintegration of a circuit requiring a current storing function.

The invention also provides a display device having a pixel providedwith a current-driving type of displaying element, the above pixelcomprising a writing element for selecting input of a video signal in aform of a current value for the pixel and a driving element forcontrolling the amount of electric current flowing to the displayingelement, wherein a semiconductor element is used in at least one of thewriting element and the driving element. The invention is especiallyeffective when the displaying element has a characteristic that itsresistant value changes in accordance with the change in temperature ortime. That is, the value of the current flowing to the displayingelement can be kept constant without depending on the change intemperature or time, so that the intensity can be kept good. It is mosteffective that the displaying element is a luminous element since theluminous element has a characteristic of depending on the change intemperature or time.

A multi-drain transistor having two drains is particularly referred toas a double drain transistor. The invention will be described mainlywith reference to an example of a pixel circuit having a current storingfunction, in which a double drain transistor of a poly-silicon thin filmis used.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will appear moreclearly upon reading the flowing detailed description, made withreference to the annexed drawings in which:

FIG. 1 is an example of a schematic structural view of a light-emittingdevice according to the invention;

FIG. 2 illustrates an example of a pixel circuit of a light-emittingdevice according to the invention;

FIG. 3 shows a timing chart of a signal inputted to a gate signal line;

FIG. 4A is a schematic view of a pixel during a writing period and FIG.4B is a schematic view of a display period;

FIG. 5 is a block diagram showing an example of a source signal linedriving circuit;

FIG. 6A is a block diagram showing an example of a source signal linedriving circuit; And FIG. 6B is an example of a source signal linedriving circuit;

FIG. 7A is a block diagram showing an example of a writing gate signalline driving circuit and FIG. 7B is a block diagram showing an exampleof an initializing gate signal line driving circuit;

FIGS. 8A-C show an example of structure of a semiconductor elementaccording to the invention;

FIGS. 9A-C show examples of structure of a semiconductor elementaccording to the invention;

FIGS. 10A-C show examples of structure of a semiconductor elementaccording to the invention;

FIGS. 11A-D show processes of producing a light-emitting deviceaccording to the invention;

FIGS. 12A-D show processes of producing a light-emitting deviceaccording to the invention;

FIGS. 13A-C show appearances of a light-emitting device according to theinvention;

FIGS. 14A-H show electronic equipments according to the invention;

FIG. 15 shows an example of a pixel circuit of a light-emitting deviceaccording to the invention;

FIG. 16 shows an example of structure of a semiconductor elementaccording to the invention;

FIG. 17 shows an example of structure of an electric circuit accordingto the invention; and

FIGS. 18A and 18B show examples of connecting three nodes by means of aconventional TFT.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode 1

FIG. 2 shows an example of a pixel circuit with a current storingfunction in a light-emitting device according to the invention.

A pixel 201 shown in FIG. 2 has a source signal line Si (one of S1 toSx), a writing gate signal line Pj (one of P1 to Py), an initializinggate signal line Ej (one of E1 to Ey) and a power supply line Vi (one ofV1 to Vx). The pixel 201 also has a writing element 101, a drivingelement 102, an initializing element 103, a capacitance element 104 anda luminous element 105.

The initializing element 103 is added in FIG. 2 because of its utilityalthough not necessarily required to the invention. In some cases, thecapacitance element 104 can be substituted by such as a parasiticcapacity without being provided explicitly.

An element or a circuit may be attached, if necessary, other than thedriving element, the writing element, the initializing element, thecapacitance element and the luminous element.

In the invention, at least one of the driving element and the writingelement is a multi-drain transistor. The both of them, however, may notbe necessarily multi-drain transistors, and one of them may be anordinary transistor (which is referred to as a single drain transistorhereinafter in the case that the discrimination is especially required).FIG. 15 shows an example in which only a driving element is amulti-drain transistor.

In FIG. 2, double drain transistors is used for both of the drivingelement and the writing element. Any one of drains of a double draintransistor is referred to as a first drain while the other is referredto as a second drain for the purpose of discrimination. It is notdefined which drain is referred to as the first drain and which drain isreferred to as the second drain. The first and second drains can beoptionally and individually designated. It is sometimes difficult todiscriminate among the source, the first drain and the second drain,depending on a way for using. In such a case, the first and seconddrains including the source are optionally designated.

The length and the width of each channel portion communicating with thesource, the first drain and the second drain in a double draintransistor may all be optional and not be necessary to be equal orsymmetrical. The channels are referred to as a source channel, a firstdrain channel and a second drain channel, respectively, hereinafter.These three channels are simply referred to as whole channels of adouble transistor, collectively. The length and the width of eachchannel can be optionally determined in accordance with a purpose ofuse.

In this embodiment mode, all channels of a double drain transistor ofthe writing element 101 (simply referred to as a writing element,hereinafter) are of an n-type, all channels of a double drain transistorof the driving element 102 (simply referred to as a driving element,hereinafter) are of a p-type and a channel of the initializing element103 is of the n-type. The writing element 101 and the initializingelement 103 may be of the p-channel type. Furthermore, the all channelsof the driving element 102 may be of the n-type. Moreover, all channelsof a double transistor per se may not necessarily of a same type,originally.

The double drain transistor can control the connection of three nodes bymeans of voltage applied to a gate electrode. A gate electrode of thewriting element 101 is connected to a writing gate signal line Pj. Thesource, the first drain and the second drain of the writing element 101are connected to a source signal line Si, a drain of the initializingelement 103 and the first drain of the driving element 102, respectively(refer to FIG. 4A). A switching element 101 has a function ofcontrolling writing of a signal into a pixel 201.

A gate electrode of the driving element 102 is connected to a drainregion of the initializing element 103. A source region, a first drainregion and a second drain region of the driving element 102 areconnected to a power supply line Vi, a second drain of the writingdevice 102 and a pixel electrode of the luminous element 105,respectively. The driving element 102 has a function of controlling thecurrent flowing to the luminous element.

The luminous element 105 may be various kinds of device such as, forexample, an OLED device, an inorganic light-emitting diode device, otherlight-emitting diode devices, an inorganic EL device, other solidluminous elements, an FED device and other vacuum luminous elements. AnOLED device is used as the luminous element 105 in this embodiment mode.The OLED device has an anode, a cathode and an organic luminous layersandwiched between the anode and the cathode.

In this embodiment mode, the anode of the OLED device 105 is used as apixel electrode and the cathode is used as a counter electrode. It isgenerally preferable to use the anode as a pixel electrode and thecathode as a counter electrode when all channels of the driving element102 are of the p-type. On the contrary, it is preferable to use thecathode as a pixel electrode and the anode as a counter electrode in thecase that all channels of the driving element 102 are of the n-type. Theusage of the anode and the cathode is not limited to the and another wayof use may be possible.

An OLED device 105 can be produced by using a known luminous material toform an organic luminous layer. The organic luminous layer has variouskinds of structure such as a single layer structure, a laminated layerstructure and an intermediate structure, either of which may be used forthe invention so long as it is known. There are two kinds ofluminescence in the organic luminous layer: luminescence (fluorescence)in returning from a single term excitation state to a base state; andluminescence (phosphorescence) in returning from a triple termexcitation state to a base state. Both of the luminescence can beapplied to the invention.

A gate electrode of the initializing element 103 is connected to aninitializing gate signal line Ej. A source of the initializing element103 is connected to a gate electrode of the driving element 102 and adrain is to the power supply line Vi.

One of two electrodes included in the capacitance element 104 isconnected to the power supply line Vi while the other is connected tothe gate of the driving element 102. More particularly, the twoelectrodes included in the capacitance element 104 are connected to thegate of the driving element 102 and the source of the driving element102.

The voltage of the power supply line Vi (power supply voltage) and thevoltage of the counter electrode are kept at constant values in thisembodiment mode. This is for the purpose of simple description and notoriginally necessary. The power supply voltage can be changed so as toapply backward bias to the luminous element 105 for a certain period inorder to improve reliability of the luminous element.

The pixel electrode is an anode of an OLED device in this embodimentmode, and therefore, the voltage of the counter electrode should be apredetermined value lower than the power supply voltage. In the casethat the pixel electrode is a cathode of an OLED device, the voltage ofthe counter electrode should be a predetermined value higher than thepower supply voltage.

FIG. 1 shows a schematic view of a whole structure of the light-emittingdevice according to the invention to which the pixel 201 shown in FIG. 2is mounted. 200 denotes a pixel portion in which pixels with a circuitshown in FIG. 2 form a matrix. 202 denotes a source signal line drivingcircuit, 203 denotes a writing gate signal line driving circuit and 204denotes an initializing gate signal line driving circuit.

There are a source signal line driving circuit 202, a writing gatesignal line driving circuit 203 and an initializing gate signal linedriving circuit 204 provided in ones in FIG. 1, but the invention is notlimited to such a structure. It is possible to optionally set the numberof the source signal line driving circuit 202, the writing gate signalline driving circuit 203 and the initializing gate signal line drivingcircuit 204 in accordance with the structure of the pixel 201. In thecase of the structure that the pixels are provided with no initializingelement 103 (FIG. 1), for example, the second writing gate signal linedriving circuit 203 may be provided instead of the initializing gatesignal line driving circuit 204.

The source signal line driving circuit 202, the writing gate signal linedriving circuit 203 and the initializing gate signal line drivingcircuit 204 can be mounted on a sheet of a glass substrate by using apoly-silicon TFT. All or a part of them, however, may be formed on asubstrate different from the pixel portion 200 (a chip, etc.) so as tobe connected to the pixel portion 200 through a connector such as a FPC.

The pixel portion 200 is provided with the source signal lines S1 to Sx,the power supply lines V1 to Vx, the writing gate signal lines P1 to Pyand the initializing gate signal lines E1 to Ey, although they are notshown in FIG. 1. The number of the source signal lines S1 to Sx is notnecessarily same as that of the power supply lines V1 to Vx. The numberof the writing gate signals P1 to Py is not necessarily same as that ofthe initializing gate signal lines E1 to Ey. Such wiring is notnecessarily be all provided. The different wiring may be provided otherthan the wiring.

The power supply lines V1 to Vx are kept at a predetermined voltage. Inthis embodiment mode, a structure of a light-emitting device displayinga monochrome image is described, but the invention may be alight-emitting device displaying a color image. In the latter case, theheight of the voltage of the power supply lines V1 to Vx all may not bekept at the same so as to change it by every color correspondingthereto.

Next, a way of driving the above-mentioned light-emitting deviceaccording to the invention will be described, made with reference toFIGS. 3 and 4. An operation of the light-emitting device according tothe invention can be divided by every pixel of each line into a writingperiod Ta, a displaying period Td, an initializing period Te and anon-displaying period Tu for the purpose of description. FIG. 3 shows atiming chart of a writing gate signal line and an initializing gatesignal line. The writing gate signal line and the initializing gatesignal are collectively referred to as a gate signal line in thisspecification. The period during which the gate signal line is selected,that is, which all of semiconductor elements having a gate electrodeconnected to the gate signal line are in the on state, is shown as ON.On the contrary, the period during which the gate signal line is notselected, that is, which all of semiconductor elements having a gateelectrode connected to the gate signal line are in the off state, isshown as OFF.

FIG. 3 briefly shows a timing chart of the writing period Ta, thedisplaying period Td, the initializing period Te and the non-displayingperiod Tu for the pixel 201. There are discrimination between thewriting period Ta and the displaying period Td and between theinitializing period Te and the non-displaying period Tu in thisembodiment mode, but the invention is not limited to the above. Theinitializing period Te may be included in the displaying period Td aswell as the writing period Ta may be included in the non-displayingperiod Tu. FIG. 4A shows a way of current flow in the pixel 201 duringthe writing period Ta. FIG. 4B shows a way of current flow in the pixel201 during the displaying period Td. The arrows shown in the pixel 201indicate the direction of the flow.

First, upon starting the writing period Ta for a first line of pixel,the writing gate signal line P1 is selected so that the writing element101 would turn on. The initializing element 103 is in the off statesince an initializing gate signal line E1 is not selected at that time.Then, the current flows between the source signal lines S1 to Sx and thepower supply lines V1 to Vx through the writing element 101 and thedriving element 102 on the basis of a video signal inputted from thesource signal line driving circuit 202 to the pixel 201.

The current flowing in the pixel 201 during the writing period Ta willbe described in more detail with reference to FIG. 4A. A gate of thewriting element 101 opens so as to turn the writing element 101 on whenthe writing gate signal line P1 is selected. A gate and a first drain ofthe driving element 102 are then short-circuited, which results in anoperation of the portion of combination between the source channel andthe first drain channel as a diode.

A source, an n-th drain, a channel portion (a source channel and then-th drain channel) between the source and the n-th drain and a gate ofa double drain transistor are hereinafter referred to as the n-thelement transistor of the double drain transistor for convenience. Asource, a first drain, a channel portion therebetween and a gate of thedriving element 102 are referred to as the first element transistor ofthe driving element 102.

When the first element transistor of the driving element 102 operates asa diode, the video signal current inputted from the source signal lineSi to the pixel 201 flows as it is to the power supply lines V1 to Vxthrough the writing element 101 and the first element transistor of thedriving element 102. At the same time, the gate voltage of the firstelement transistor of the driving element 102, which corresponds to thevideo signal current inputted from the source signal line Si to thepixel 201, is accumulated in the capacitance element 104 through thewriting element 101. The voltage accumulated in the capacitance element104 is the voltage V_(GS) between the gate and the source of the firstelement transistor of the driving element 102, and therefore, the firstelement transistor of the driving element 102 turns on in accordancewith the voltage of the capacitance element 104.

The first element transistor of the driving element 102 operates in asaturable region during the writing period Ta since its gate and drainare connected to each other. Thus, when V_(GS) is the voltage betweenthe gate and the source, μ is the degree of movement, C_(o) is the gatecapacity per a unit area, W/L is the ratio of the channel width W andthe channel length L in a channel forming region and V_(TH) is thethreshold, the drain current I_(dn) of the first element transistor ofthe driving element 102 is expressed by the following formula:I _(dn) =μC _(o) W/L(V _(GS) −V _(TH))²/2.

In the above formula, all of μ, C_(o), W/L and V_(TH) are fixed valuesdetermined in accordance with the individual element. Therefore, when μand V_(TH) are dispersed among the respective elements, the values ofall the elements would not be always equal even in the case of I_(dn)for the same V_(GS). Maintaining the drain current I_(dn) of the firstelement transistor of the driving element 102 to be equal to the videosignal current I_(vd), however, can make I_(dn) equal for all of thefirst element transistor of the driving element 102 regardless ofdispersion of μ and V_(TH).

The selection of the writing gate signal line P1 is completed upon thecompletion of the writing period Ta for the first line of pixel. Thedisplaying period Td is then started. The writing element 101 is in theoff state since the writing gate signal line P1 is not selected duringthe displaying period Td. The initializing element 103 is also in theoff state since the initializing gate signal line E1 is not selectedduring the displaying period Td.

The flow of the current in the pixel 201 during the display period Tdwill be described with reference to FIG. 4B. V_(GS) determined duringthe writing period Ta is held by the capacitance element 104 in the gateelectrode of the driving element 102. The current, however, does notflow toward the first element transistor of the driving element 102 andflows to the luminous element through the second element transistorsince the writing element 101 is in the off state during the displayingperiod Td.

The second element transistor of the driving element 102 operates in thesaturated region. In order to satisfy such a condition, it is requiredto properly set in advance the video signal current I_(VD) to be writteninto the pixel and the counter electrode voltage.

The drain current I_(dn) of the second element transistor of the drivingelement 102 is expressed by I_(dn)=μC_(o)W/L(V_(GS)−V_(TH))²/2 becauseof the operation in the saturable region. In accordance withI_(dn)=μC_(o)W/L(V_(GS)−V_(TH))²/2, the drain current I_(dn) must dependon the value of μ, V_(TH) or such. On the other hand, V_(GS) is set inwriting so that the drain current I_(dn) of the first element transistorwould be I_(vd). Therefore, the dispersion of μ, V_(TH) and such betweenthe second element transistors of the driving element 102 in each pixelis not reflected in the drain current I_(dn) of the second elementtransistor when the values of μ, V_(TH) and such of the first elementtransistor and the second element transistor are equal in the drivingelement 102 in each pixel.

That is to say, the necessity for controlling the dispersion of μ,V_(TH) and such can be greatly reduced from the driving element 102 ofpixels in a whole screen in the light-emitting device to a portionbetween the first element transistor and the second element transistorof the driving element 102 in each pixel. Furthermore, the dispersion ofμ, V_(TH) and such of the first element transistor and the secondelement transistor is originally little in a double drain transistor.

Thus, the drain current I_(dn) of the second element transistor of thedriving element 102 appropriately corresponds to the video signalcurrent I_(vd) during the displaying period Td. Namely, thepredetermined proper current flows from the power supply line Vi to thecounter power supply of the luminous element 105 through the secondelement transistor of the driving element 102. The current appropriatelyflowing to the luminous element 105 results in light-emission of theluminous element 105 at an appropriate intensity. The luminous element105 does not emit a light, of course, when the drain current I_(dn) iszero.

The video signal current I_(vd) should be an appropriate value of thecurrent in principle. Exception is the case that the content of thevideo signal is the darkest gradation “non-lighting”. In this case, thevideo signal is just required to turn off the element transistor of thedriving element 102, so that the data of the current values would beenough.

When the channel length and the channel width of the first drain channeland the second drain channel of the driving element are equal, the videosignal current I_(vd) read into during the writing period is equal tothe driving current I_(el) supplied to the luminous element during thedisplaying period. The ratio of the video signal current I_(vd) readinto during the writing period and the driving current I_(el) suppliedto the luminous element during the displaying period can be adjusted bysetting the channel length and the channel width of the first drain andthe second drain unequal on purpose (refer to FIG. 16).

Such adjustment of the ratio is significantly convenient in practicaluse. When display is carried out at the low intensity by means of acompact and highly sophisticated light-emitting displaying device, forexample, the driving current I_(el) supplied to the luminous elementduring the displaying period becomes extremely small. This is because,in view of a load of such as the parasitic capacity, writing into thepixels is impossible within the displaying period unless the videosignal current I_(vd) is made larger than I_(el).

When the writing period Ta is completed for the first line of pixel, thewriting gate signal line P2 is selected, so that the writing period Tafor the second line of pixel would start. The writing element 101 thenturns on in the second line of pixel. The initializing element 103 is inthe off state since an initializing gate signal line E2 is not selected.The video signal current flows between the signal lines S1 to Sx and thepower supply lines V1 to Vx through the writing element 101 and thedriving element 102 in the second line of pixel on the basis of thevideo signal inputted from the source signal line driving circuit 202 tothe pixel 201.

After the above, the writing period Ta is completed for the second lineof pixel so as to start the displaying period Td. In the displayingperiod Td, the video signal current I_(vd) is also stored in the secondline of pixel as well as the case of the first line of pixel, and theluminous element 105 emits a light at the predetermined intensity. Whenthe writing period Ta is completed for the second line of pixel, startsthe writing period Ta for the third line of pixel.

Then, the operation is repeated such that the displaying period Td forthe third line of pixel starts at the same time as starting of thewriting period Ta for the fourth line of pixel, the writing period Ta iscompleted for the fourth line of pixel, and then, the displaying periodTd for the fourth line of pixel starts at the same time as starting ofthe writing period Ta for the fifth line of pixel, . . . . The wholewriting period for a frame is completed when the writing period Ta iscompleted until the y-th line, that is the final line, of pixel in turn.

In sight, an image during the displaying period Td for a frame isoverlapped in the time so as to be caught as unified one. Therefore,image display is possible for a frame during the whole displaying periodTd for a frame. Typically, in dynamic image display, an image isdisplayed for 60 Hz driving, that is, for 60 frames per a second.

The above is a whole operation without any initializing element 103. Inthe case that there is any initializing element 103, it is possible tofurther add the following initializing operation. An image for eachframe is displayed in series when the initializing operation does notexist, so that there would be a problem that the dynamic image displayis somewhat deteriorated such that smooth movement of an image is notenough. Such deterioration of the dynamic image quality can be easilyand effectively restrained by providing a non-display interval betweenrespective frames in the initializing operation.

The initializing operation is controlled by a gate signal outputted fromthe initializing gate signal line driving circuit 204. First, the firstline of the initializing gate signal line E1 is selected in accordancewith the gate signal outputted from the initializing gate signal linedriving circuit 204 so as to start the initializing period Te for thefirst line of pixel. The initializing element 103 turns on uponselection of the initial gate signal line E1. The voltage of the powersupply line V1 to Vx is then applied to a gate electrode of the drivingelement 102 through the initializing element 103. The driving element102 is thereby forced to turn off not to supply the luminous element 105with the current, so that the luminous element 105 would stop emitting alight.

Then, the initializing gate signal to be selected moves from E1 in thefirst line to E2 in the second line. The initializing period Te isthereby completed for the first line and the non-displaying period Tustarts. At the same time, start the initializing period Te for thesecond line of pixel.

The initializing element 103 turns on in the second line of pixel uponselection of the initializing gate signal line E2. The writing element101 is in the off state since the writing gate signal line P2 is notselected. The voltage of the constant power supply lines V1 to Vx isadded at that time to a gate electrode of the driving element 102through the initializing element 103. The driving element 102 then turnsoff not to supply the luminous element 105 with the current, so that theluminous element 105 would stop emitting a light.

The initializing gate signal line to be selected moves from E2 in thesecond line to E3 in the third line after the above. The operation is sorepeated that the initializing period Te starts in turn until the y-thline of pixel and a non-displaying period Tu starts after the completionof the initializing period Te. The initializing operation is thuscarried out in all of the pixels.

In the case that the initializing operation is carried out with theinitializing element 103, a frame period comprises the writing periodTa, the displaying period Td, the initializing period Te and thenon-displaying period Tu and an image for a frame is displayed. Aftercompletion of a frame period, next frame period starts to repeat theabove-mentioned operation. The dynamic image quality can be easily andeffectively improved by providing a non-display interval betweenrespective frames in the initializing operation. The initializing periodTe and the non-displaying period Tu are not necessarily provided withina frame period. For example, it is possible to omit the initializingperiod Te and the non-displaying period Tu for a static image and to setthe initializing period Te and the non-displaying period Tu only for adynamic image.

The gradation of each pixel is determined in accordance with the amountof the current flowing to the luminous element 105 during the writingperiod Ta and the displaying period Td. The current value is controlledin accordance with the video signal current I_(vd) inputted from thesource signal line driving circuit 202 to the pixel 201. It means thatthe video signal current I_(vd) for n gradations enables image displayfor n gradations. Generally, it is said that the luminous intensity L ofan OLED device is proportional to the current amount I (V) flowing tothe OLED device, as expressed by L=c (V) I (V). The video signal currentI_(vd) for n gradations has, therefore, n values proportionallyapportioned in general.

As described hereinbefore, the pixel circuit structure shown in FIG. 2can maintain the current flowing to a luminous element such as an OLEDdevice in the light-emitting device well even in the following cases:the case that the electric resistance of the luminous element depends onambient temperature; and the case that the voltage-driving of theluminous element lowers the luminous intensity as the time elapses.Maintaining the current flowing in the luminous element well enables theluminous intensity to be kept good. As a result, color shift can be alsoavoided in a color display device of the type that respective sub-pixelsin RGB are separately formed.

The current-driving of the luminous element in the pixel circuitstructure shown in FIG. 2 enables significant difference between pixelsin the amount of the current flowing to the luminous element to beprevented from occurring even when a characteristic of the drivingelement 102 for controlling the current flowing to the luminous elementis different between pixels, so that the uneven intensity of a displayscreen can be also restrained.

Furthermore, the current flowing to the luminous element can be kept atthe desired value, so that change of gradation due to potential fallcaused by wiring resistance can be prevented. This is also an advantagein comparison with the voltage-driving of the luminous element.

A multi-drain transistor is a new device capable of being usedeffectively for a circuit, the circuit which is difficult to onlycomprise a single drain transistor, or which is possible to onlycomprise a single drain transistor but may be complicated or require thelarge area. An example of the multi-drain transistor is a pixel circuitof a light-emitting device comprising a writing element and a drivingelement as shown in FIGS. 2 and 15, in which a multi-drain transistor isused for one or both of the two elements.

A current storing circuit comprising a writing element and a drivingelement in which a multi-drain transistor is used for one or both of thetwo elements (an example of which is shown in FIG. 17) can be widelyused not only for a pixel circuit of a light-emitting device but alsofor a current signal buffer. For example, a current signal buffer usingthe current storing circuit with a multi-drain transistor can beprovided in the source signal line driving circuit 202 (FIG. 1) of alight-emitting device.

In some cases, it is possible to apply the current storing circuit to adisplaying device using a non-luminous element rather than a luminouselement shown in FIG. 2.

Embodiment Mode 2

In Embodiment Mode 1, described an example of each of a semiconductormulti-drain transistor, a current storing circuit using the multi-draintransistor and a light-emitting device using the current storing circuitin a pixel according to the invention. In the light-emitting devicedescribed in Embodiment Mode 1, the video signal has an analog currentvalue (referred to as analog driving, hereinafter), but it is possibleto use the video signal digitally for driving (referred to as digitaldriving, hereinafter).

In the case of using a digital video signal, the gradation is coded inthe binary number to be inputted. It is easy and effective as a methodof displaying the gradation to write into a pixel the coded binary videosignal as it is and to control the time or area for emitting a light inaccordance with the binary code while the intensity of an emitted lightis kept constant. In Embodiment Mode 2, an example of a method ofcontrolling the time for emitting a light in accordance with a binarycode (a digital time gradation method) is described briefly. It ispossible to refer to Japanese Patent Application No. 359032/2000 orfurther detail.

In Embodiment Mode 2, used a pixel circuit shown in FIG. 2. In thedigital time gradation method, it is possible to display an image byrepeating the writing period Ta and the displaying period Td in a frameperiod.

In the case that an image is displayed by means of n bits of videosignal, for example, at least n writing periods and n displaying periodsare provided in a frame period. The n writing periods (Ta1 to Tan) andthe n displaying periods (Td1 to Tdn) correspond to respective bits ofthe video signal.

Furthermore, it is possible, but not necessary, to provide theinitializing periods not more than n and the non-displaying periods notmore than n in a frame period. It is significantly difficult, in view ofthe current TFT manufacturing technology, to manufacture a practicaldisplay device and light-emitting device in which a signal line drivingcircuit is mounted on a glass substrate, without providing theinitializing period and the non-displaying period for at least lowerbits. For the further detail, refer to Japanese Patent Application No.257163/2001.

After the writing period Tam (wherein m is any number from 1 to m),appears a displaying period Tdm corresponding to the above-mentionedbit. In the case that the initializing period Tem and the non-displayingperiod Tum are provided for the bit, appears a subsequent initializingperiod Tem and non-displaying period Tum. A series of periods comprisingthe writing period Ta, the displaying period Td, the initializing periodTe and the non-displaying period Tu (as for the initializing period Teand the non-displaying period Tu, only the case of existence) isreferred to as sub-frame period SF. A sub-frame period including thewriting period Tam and the displaying period Tdm corresponding to them-th bit is SFm.

The rate of the length of the sub-frame periods SF1 to SFn shouldsatisfy SF1: SF2: . . . :SFn=2(0): 2(1): . . . : 2(n−1).

In each sub-frame period, whether the luminous element is made emit alight or not is selected by means of each bit of the digital videosignal. The total of the length of the displaying periods for emitting alight in a frame period is controlled so as to control the number ofgradations.

The sub-frame period in which the displaying period is long may bedivided into some in order to improve the image quality in display.

An operation of the pixel circuit and the driving circuit are almostsame as those of Embodiment Mode 1. The source signal line drivingcircuit is only required to accurately output a predetermined value ofthe current at the time the luminous element emits a light. As a result,it is an advantage that the structure can be greatly simplified incomparison with the case in Embodiment Mode 1 in which the analogcurrent value corresponding to the number of gradations is necessary.When a signal for stopping the luminous element from emitting a light isoutputted from the source signal line driving circuit, the data of thecurrent values would be enough as well as when a signal with zerogradation is outputted in Embodiment Mode 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

An example of the source signal line driving circuit 202 shown in FIG. 1is described in this embodiment. The source signal line driving circuit202 is capable of supplying respective source signal lines S1 to Sx withthe current corresponding in size to the voltage of the video signalinputted to the pixel 201 (signal current I_(vd)). In this embodiment,an example of the source signal line driving circuit 302 in the case ofdigital-driving is first described with reference to FIG. 5. An exampleof the source signal line driving circuit 402 in the case ofanalog-driving is secondly described with reference to FIGS. 6A and 6B,and an example of the gate signal line driving circuit is thirdlydescribed with reference to FIGS. 7A and 7B.

First, an example of the source signal line driving circuit 302 in thecase of digital-driving is described with reference to FIG. 5. Thesource signal line driving circuit 302 has a shift register 302 a, alatch (A) 302 b capable of storing the digital video signal, a latch (B)302 c and a voltage-current converter circuit (a V/C converter circuit)302 d.

A clock signal (CLK) and a start pulse signal (SP) are inputted to theshift register 302 a. The shift register 302 a generates in turn timingsignals for sampling the video signals on the basis of the clock signal(CLK) and the start pulse signal (SP). The latch (A) 302 b reads fromthe video signal line and stores the video signals on the basis of therespective timing signals.

The video signal stored in the latch (A) 302 b is read by the latch (B)302 c in accordance with the timing of a latch pulse so as to be stored.After the latch (B) 302 c reads the data, the V/C converter circuit 302d outputs predetermined current data, when the read data indicates ON.When the read data indicates OFF, it is possible to output otherpredetermined current data, but is more efficiently and preferable tooutput the voltage data.

In the digital driving method, the driving operation is carried outunder the following two states of the luminous element: the on state (inwhich the brightness is 100%); and the off state (in which thebrightness is 0%). According to the above-mentioned structure of thesource signal line driving circuit, the digital driving light-emittingdevice expresses gradation by switching the luminous element between theon and off states.

Next, an example of the source signal line driving circuit 402 in thecase of analog-driving is described with reference to FIGS. 6A and 6B.The source signal line driving circuit 402 shown in FIG. 6A in thisembodiment has a shift register 402 a, a buffer 402 b, a samplingcircuit 402 c and a current converter circuit 402 d.

A clock signal (CLK) and a start pulse signal (SP) are inputted to theshift register 402 a. The shift register 402 a generates in turn timingsignals for sampling the video signals on the basis of the clock signal(CLK) and the start pulse signal (SP).

The timing signal is buffered and amplified in the buffer 402 b to beinputted to the sampling circuit 402 c. It is possible to provide alevel shifter instead of the buffer to amplify in voltage the timingsignal, if necessary. Both of the buffer and the level shifter may beprovided, and on the contrary, none of them may be provided so as not toespecially amplify the timing signal.

The sampling circuit 402 c reads in the video signal on the basis of therespective timing signals amplified in response to the necessity so asto transmit the video signal to the V/C converter circuit.

FIG. 6B shows a concrete structure of the sampling circuit 402 c and thecurrent converter circuit 402 d. The sampling circuit 402 c is connectedat a terminal 410 to an outputting portion of the buffer 402 b.

The sampling circuit 402 c is provided with a plurality of switches 411.Each switch 411 samples analog video signals from the video signal line406 synchronously with the timing signal so as to transmit the sampledsignals to the subsequent current converter circuit 402 d. In FIG. 6B,only a current converter circuit connected to one of the switch 411included in the sampling circuit 402 c is shown as the current convertercircuit 402 d. There are other current converter circuits 402 d as shownin FIG. 6B, however, connected behind the respective switches 411.

Only one transistor is used as the switch 411 in this embodiment, butthe structure is not limited to this embodiment as long as the switch411 is just a switch capable of sampling analog video signalssynchronously with the timing signal.

The sampled analog video signal is inputted to a current outputtingcircuit 412 included in the current converter circuit 402 d. The currentoutputting circuit 412 outputs the current corresponding to the inputtedvideo signal voltage (signal current I_(vd)). The current outputtingcircuit comprises an amplifier and a transistor in FIGS. 6A and 6B. Theinvention is, however, not limited to the above structure and it is onlyrequired that the circuit is able to output the current having a valuecorresponding to the inputted video signal.

The signal current I_(vd) is inputted to a reset circuit 417 alsoincluded in the current converter circuit 402 d. The reset circuit 417has two analog switches 413 and 414, an inverter 416 and a power supply415.

The analog switch 414 is controlled by a reset signal (Res) while theanalog switch 413 is controlled by a reset signal (Res) inverted by theinverter 416. Therefore, the analog switch 413 and the analog switch 414operate synchronously with the inverted reset signal and the formerreset signal, respectively, so that one would turn off when the otherturns on.

When the analog switch 413 is in the on state, the signal current isinputted to the source signal line. On the other hand, the voltage ofthe power supply 415 is added to the source signal line so as to resetthe source signal line when the analog switch 414 is in the on state.The voltage of the power supply 415 is preferably almost same as that ofthe power supply line provided in the pixel. The closer to zero thecurrent flowing to the source signal line is in a reset state of thesource signal line, the better.

The source signal line is preferably reset during a flyback period. Itis possible, however, to reset the source signal line during a periodother than the flyback period, if necessary, so long as an image is notdisplayed during the period.

Further, it is possible to use another circuit such as a decodercircuit, for example, instead of the shift register so as to be able toselect the source signal line.

Now, the structure of the writing gate signal line driving circuit 203and the initializing gate signal line driving circuit 204 will bedescribed with reference to FIGS. 7A and 7B.

FIG. 7A is a block diagram showing the structure of the writing gatesignal line driving circuit 203. The writing gate signal line drivingcircuit 203 has a shift register 203 a and a buffer 203 b. In somecases, the writing gate signal line driving circuit 203 may further havea level shifter and may have no buffer 203 b.

In the writing gate signal line driving circuit 203, the timing signalsare generated in turn by inputting the clock signal CLK and the startpulse signal SP to the shift register 203 a. The respective generatedtiming signals are buffered and amplified by the buffer 203 b so as tobe supplied to the corresponding writing gate signal line.

The writing gate signal line is connected to a gate electrode of thewriting element 101 in the pixels for a line. The buffer 203 b is usedfor flowing a large amount of current since the writing element 101 inthe pixels for a line should be simultaneously turned on.

FIG. 7B is a block diagram showing the structure of the initializinggate signal line driving circuit 204. The initializing gate signal linedriving circuit 204 has a shift register 204 a and a buffer 204 b. Insome cases, the initializing gate signal line driving circuit 204 mayfurther have a level shifter and may have no buffer 204 b.

In the initializing gate signal line driving circuit 204, the timingsignals are generated in turn by inputting the clock signal CLK and thestart pulse signal SP to the shift register 204 a. The respectivegenerated timing signals are buffered and amplified by the buffer 204 bso as to be supplied to the corresponding initializing gate signal line.

The initializing gate signal line is connected to a gate electrode ofthe initializing element 103 in the pixels for a line. The buffer 204 bto be used is capable of flowing a large amount of current since theinitializing element 103 in the pixels for a line should besimultaneously turned on.

Further, it is possible to use another circuit such as a decodercircuit, for example, instead of the shift register so as to be able toselect the gate signal line (a scanning line).

The structure of the writing gate signal line driving circuit 203 andthe initializing gate signal line driving circuit 204 may be differentalthough it is same in this embodiment. The source signal line drivingcircuit, the writing gate signal line driving circuit and theinitializing gate signal line driving circuit for driving thelight-emitting device in accordance with the invention are not limitedto the structure described in this embodiment.

The structure in this embodiment can be practically carried out inoptional combination with either or both of Embodiment Modes 1 and 2.

Embodiment 2

An example of a semiconductor element to be used for a light-emittingdevice in accordance with the invention is described in this embodimentwith reference to FIGS. 8A-C. FIG. 8A is a top view of a semiconductorelement according to the invention. FIG. 8B is a sectional view taken bya broken line A-A′ shown in FIG. 8A. FIG. 8C is a sectional view takenby a broken line B-B′ shown in FIG. 8A.

The semiconductor element according to the invention has a semiconductorlayer 501, a gate insulating film 502 contacting with the semiconductorlayer 501 and a gate electrode 503 contacting with the gate insulatingfilm 502. The semiconductor layer 501 has a channel forming region 504and impurity regions 505, 506 and 507 to which impurity for givingconductivity is added. A typical example of the impurity is boron forthe p-channel type and phosphorous for the n-channel type. The gateelectrode 503 and the channel forming region 504 sandwich the gateinsulating film therebetween and overlap each other.

The impurity regions 505, 506 and 507 contact with the channel formingregion 504, respectively. In this embodiment, all of the impurityregions contact with the channel forming region 504, respectively, butthe invention is not limited to this structure. It is possible toprovide between the impurity region and the channel forming region a lowconcentration impurity region (an LDD region) in which the concentrationof the impurity is lower than that of the impurity regions and toprovide a region, which does not overlap with the gate electrode and inwhich no impurity is added (an offset region).

An insulating film 508 is formed on the gate insulating film 502 so asto cover the impurity regions 505, 506 and 507 of the semiconductorlayer 501. Connecting wirings 509, 510 and 511 are formed so as to beconnected respectively to the impurity regions 505, 506 and 507 throughcontact holes formed in the insulating film 508 and the gate insulatingfilm 502. In FIGS. 8A-C, the gate insulating film 502 covers theimpurity regions 505, 506 and 507, but the invention is not limited tothis structure. The impurity regions 505, 506 and 507 are notnecessarily covered by the gate insulating film 502 and may be exposed.

In the semiconductor element shown in FIGS. 8A-C, the voltage applied tothe gate electrode 503 simultaneously controls the resistance betweenthe respective connecting wirings 509, 510 and 511.

The simplest way of using the semiconductor element shown in FIGS. 8A-Cis to connect or open three nodes, that is, nodes 509, 510 and 511,concretely, at the same time. The connection in this specification meanselectric connection so long as there is any description separately.

The way of utilizing a multi-drain transistor, however, is not limitedto the above. For example, it is also possible to set the node 509 athigh potential, the node 510 at low potential and the node 511 at middlepotential to connect the gate electrode 503 with the node 511, andthereby, to flow the current between either one of the nodes 509 and 510and the node 511 selectively.

Generally, more than two transistors are required in order to controlconnection of three nodes by means of a single drain transistor. Anexample of the above is described in FIGS. 18A and 18B. According to theinvention, however, using a multi-drain transistor enables the totalarea, which is occupied by semiconductor elements such as a transistor,to be kept small. As a result, applying the semiconductor elementaccording to the invention to a pixel circuit of a display device can bemade highly fine or highly functionalize the display device withoutlowering the aperture rate of pixels.

The structure in this embodiment can be practically carried out inoptional combination with either or all of Embodiment Modes 1 and 2 andEmbodiment 1.

Embodiment 3

In this embodiment, a semiconductor element according to the invention,which is provided with two or more channel forming regions betweenrespective impurity regions connected to the connecting wirings andwhich has a so-called multi gate structure, is described. Thesemiconductor element to be described in this embodiment has a doublegate structure in which two channel forming regions are provided betweenthe respective connecting wirings, but the invention is not limited tothe double gate structure and may have a multi-gate structure in whichthree or more channel forming regions are provided between therespective wirings.

Now, the structure of the semiconductor element in this embodiment willbe described with reference to FIGS. 9A-C. FIG. 9A is a top view of asemiconductor element to be used for the light-emitting device accordingto the invention. FIG. 9B is a sectional view taken by a broken lineA-A′ shown in FIG. 9A. FIG. 9C is a sectional view taken by a brokenline B-B′ shown in FIG. 9A.

The semiconductor element according to the invention has a semiconductorlayer 601, a gate insulating film 602 contacting with the semiconductorlayer 601 and a gate electrodes 603 a, 603 b and 603 c contacting withthe gate insulating film 602. The gate electrodes 603 a, 603 b and 603 care electrically connected each other and all of the gate electrodesform a part of a gate wiring 613 in this embodiment. The semiconductorlayer 601 has a channel forming regions 604 a, 604 b and 604 c andimpurity regions 605, 606, 607 and 612 to which impurity for givingconductivity is added. A typical example of the impurity is boron forthe p-channel type and phosphorous for the n-channel type.

The gate electrode 603 a and the channel forming region 604 a sandwichthe gate insulating film 602 therebetween and overlap each other. Thegate electrode 603 b and the channel forming region 604 b sandwich thegate insulating film 602 therebetween and overlap each other. And thegate electrode 603 c and the channel forming region 604 c sandwich thegate insulating film 602 therebetween and overlap each other.

The impurity regions 605, 606 and 607 contact with the channel formingregion 604 a, 604 b and 604 c, respectively. The impurity region 612contacts with all of the channel forming regions 604 a, 604 b and 604 c.Thus, the two channel forming regions 604 a and 604 b are providedbetween the impurity regions 605 and 606, the two channel formingregions 604 b and 604 c are provided between the impurity regions 606and 607, and the two channel forming regions 604 c and 604 a areprovided between the impurity regions 607 and 605.

In this embodiment, all of the impurity regions contact with the channelforming regions, respectively, but the invention is not limited to thisstructure. It is possible to provide between the impurity region and thechannel forming region a low concentration impurity region (an LDDregion) in which the concentration of the impurity is lower than that ofthe impurity regions and to provide a region, which does not overlapwith the gate electrode and in which no impurity is added (an offsetregion).

An insulating film 608 is formed on the gate insulating film 602 so asto cover the impurity regions 605, 606 and 607 of the semiconductorlayer 601. Connecting wirings 609, 610 and 611 are formed so as to beconnected respectively to the impurity regions 605, 606 and 607 throughcontact holes formed in the insulating film 608 and the gate insulatingfilm 602. In FIGS. 9A-C, the gate insulating film 602 covers theimpurity regions 605, 606 and 607, but the invention is not limited tothis structure. The impurity regions 605, 606 and 607 are notnecessarily covered by the gate insulating film 602 and may be exposed.

In the semiconductor element shown in FIGS. 9A-C, the voltage applied tothe gate electrodes 603 a, 603 b and 603 c controls the resistancebetween the respective connecting wirings 609, 610 and 611.

In the semiconductor element shown in FIGS. 9A-C, three nodes, that is,connecting wirings 609, 610 and 611, concretely, can be connected at thesame time.

The above structure can keep the area of a semiconductor element small.As a result, applying the semiconductor element according to theinvention to a pixel circuit of a display device enables the areaoccupied by semiconductor elements in a pixel to be kept small, andthereby, can be made highly fine or highly functionalize the displaydevice without lowering the aperture rate of pixels. In the case ofusing a transistor with three terminals of a double gate, the connectionof three nodes is controlled as shown in FIG. 18B, for example. In thiscase, however, the area to be occupied apparently becomes larger thanthat of a switching element shown in FIG. 9A.

Furthermore, in the multi-gate structure, the off-current can be madefurther microscopic in comparison with the single gate structure.Therefore, the multi-gate structure is more proper in the case of usinga transistor as a switching element.

The structure in this embodiment can be practically carried out inoptional combination with either or all of Embodiment Modes 1 and 2 andEmbodiments 1 and 2.

Embodiment 4

In this embodiment, a bottom gate type of semiconductor elementaccording to the invention, which is provided with a gate electrodebetween a substrate and a semiconductor layer thereof is described.

Now, the structure of the semiconductor element in this embodiment willbe described with reference to FIG. 10. FIG. 10A is a top view of asemiconductor element according to the invention. FIG. 10B is asectional view taken by a broken line A-A′ shown in FIG. 10A. FIG. 10Cis a sectional view taken by a broken line B-B′ shown in FIG. 10A.

The semiconductor element according to the invention has a gateelectrode 701, a gate insulating film 702 contacting with the gateelectrode 701 and an active layer 703 contacting with the gateinsulating film 702. The semiconductor layer 703 has a channel formingregion 704 and impurity regions 705, 706 and 707 to which impurity forgiving conductivity is added. The gate electrode 701 and the channelforming region 704 sandwich the gate insulating film 702 therebetweenand overlap each other. 708 denotes a mask to be used in forming thechannel forming regions, the mask which is made of an insulating film.

The impurity regions 705, 706 and 707 contact with the channel formingregion 704, respectively. In this embodiment, all of the impurityregions contact with the channel forming region 704, respectively, butthe invention is not limited to this structure. It is possible toprovide between the impurity region and the channel forming region a lowconcentration impurity region (an LDD region) in which the concentrationof the impurity is lower than that of the impurity regions and toprovide a region, which does not overlap with the gate electrode and inwhich no impurity is added (an offset region).

An insulating film 708 is formed so as to cover the impurity regions705, 706 and 707 of the semiconductor layer 703. Connecting wirings 709,710 and 711 are formed so as to be connected respectively to theimpurity regions 705, 706 and 707 through contact holes formed in theinsulating film 708.

In the semiconductor element shown in FIG. 10, the voltage applied tothe gate electrode 701 controls the resistance between the respectiveconnecting wirings 709, 710 and 711.

In the semiconductor element shown in FIG. 10, three nodes, that is,connecting wirings 709, 710 and 711, concretely, can be connected at thesame time.

The above structure can keep the area of a semiconductor element small.As a result, applying the semiconductor element according to theinvention to a pixel circuit of a display device can be made highly fineor highly functionalize the display device without lowering the aperturerate of pixels.

It is also possible to provide two or more channel forming regionsbetween the respective connecting wirings so as to form a multi-gatestructure.

The structure in this embodiment can be practically carried out inoptional combination with either or all of Embodiment Modes 1 and 2 andEmbodiments 1 to 3.

Embodiment 5

An example of a method of manufacturing the light emitting device of thepresent invention will be described with a reference of FIGS. 11A to 11Dand 12A to 12D. In this embodiment, a method of manufacturing the lightemitting device having pixels as shown in FIG. 2. Note that, a drivingelement 102 and an initializing element 103 will be representativelyexplained here. Although writing element 101 is not illustratedparticularly, it is possible that the writing element 101 ismanufactured in accordance with the manufacturing method in thisembodiment.

Although this embodiment shows an example of the light emitting deviceusing an OLED element as a light emitting element, a light emittingdevice wherein only the light emitting element is substituted by otherlight emitting element can be manufactured.

First, as shown in FIG. 11A, a base film 5002 formed of an insulatingfilm such as a silicon oxide film, a silicon nitride film or a siliconoxynitride film is formed on a substrate 5001 formed of glass such asbarium borosilicate glass or alumino borosilicate glass represented by#7059 glass and #1737 glass of Coning Corporation. For example, asilicon oxynitride film 5002 a formed from SiH₄, NH₃ and N₂O by theplasma CVD method and having a thickness of from 10 to 200 nm(preferably 50 to 100 nm) is formed. Similarly, a hydrogenerated siliconoxynitride film 5002 b formed from SiH₄ and N₂O and having a thicknessof from 50 to 200 nm (preferably 100 to 150 nm) is layered thereon. Inthis embodiment, the base film 5002 has a two-layer structure, but mayalso be formed as a single layer film of one of the insulating films, ora laminate film having more than two layers of the insulating films.

Island-like semiconductor layers 5005 to 5006 are formed from acrystalline semiconductor film obtained by conducting lasercrystallization method or a known thermal crystallization method on asemiconductor film having an amorphous structure. These island-likesemiconductor layers 5005 to 5006 each has a thickness of from 25 to 80nm (preferably 30 to 60 nm). No limitation is put on the material of thecrystalline semiconductor film, but the crystalline semiconductor filmis preferably formed from silicon, a silicon germanium (SiGe) alloy,etc.

When the crystalline semiconductor film is to be manufactured by thelaser crystallization method, an excimer laser, a YAG laser and an YVO₄laser of a pulse oscillation type or continuous light emitting type areused. When these lasers are used, it is preferable to use a method inwhich a laser beam radiated from a laser oscillator is converged into alinear shape by an optical system and then is irradiated to thesemiconductor film. A crystallization condition is suitably selected byan operator. When the excimer laser is used, pulse oscillation frequencyis set to 300 Hz, and laser energy density is set to from 100 to 400mJ/cm² (typically 200 to 300 mJ/cm²). When the YAG laser is used, pulseoscillation frequency is preferably set to from 30 to 300 kHz by usingits second harmonic, and laser energy density is preferably set to from300 to 600 mJ/cm² (typically 350 to 500 mJ/cm²). The laser beamconverged into a linear shape and having a width of from 100 to 1000 μm,e.g. 400 μm, is irradiated to the entire substrate surface. At thistime, overlapping ratio of the linear laser beam is set to from 50 to90%.

Note that, a gas laser or solid state laser of continuous oscillationtype or pulse oscillation type can be used. The gas laser such as anexcimer laser, Ar laser, Kr laser and the solid state laser such as YAGlaser, YVO₄ laser, YLF laser, YAlO₃ laser, glass laser, ruby laser,alexandrite laser, Ti: sapphire laser can be used as the laser beam.Also, crystals such as YAG laser, YVO₄ laser, YLF laser, YAlO₃ laserwherein Cr, Nd, Er, Ho, Ce, Co, Ti or Tm is doped can be used as thesolid state laser. A basic wave of the lasers is different depending onthe materials of doping, therefore a laser beam having a basic wave ofapproximately 1 μm is obtained. A harmonic corresponding to the basicwave can be obtained by the using non-linear optical elements.

When a crystallization of an amorphous semiconductor film is conducted,it is preferable that the second harmonic through the fourth harmonic ofbasic waves is applied by using the solid state laser which is capableof continuous oscillation in order to obtain a crystal in large grainsize. Typically, it is preferable that the second harmonic (with athickness of 532 nm) or the third harmonic (with a thickness of 355 nm)of an Nd:YVO₄ laser (basic wave of 1064 nm) is applied. Specifically,laser beams emitted from the continuous oscillation type YVO₄ laser with10 W output is converted into a harmonic by using the non-linear opticalelements. Also, a method of emitting a harmonic by applying crystal ofYVO₄ and the non-linear optical elements into a resonator. Then, morepreferably, the laser beams are formed so as to have a rectangular shapeor an elliptical shape by an optical system, thereby irradiating asubstance to be treated. At this time, the energy density ofapproximately 0.01 to 100 MW/cm² (preferably 01. to 10 MW/cm²) isrequired. The semiconductor film is moved at approximately 10 to 2000cm/s rate relatively corresponding to the laser beams so as to irradiatethe semiconductor film.

Next, a gate insulating film 5007 covering the island-like semiconductorlayers 5005 to 5006 is formed. The gate insulating film 5007 is formedfrom an insulating film containing silicon and having a thickness offrom 40 to 150 nm by using the plasma CVD method or a sputtering method.In this embodiment, the gate insulating film 5007 is formed from asilicon oxynitride film with a thickness of 120 nm. However, the gateinsulating film is not limited to such a silicon oxynitride film, it maybe an insulating film containing other silicon and having a single layeror a laminated layer structure. For example, when a silicon oxide filmis used, TEOS (Tetraethyl Orthosilicate) and O₂ are mixed by the plasmaCVD method, the reaction pressure is set to 40 Pa, the substratetemperature is set to from 300 to 400° C., and the high frequency (13.56MHz) power density is set to from 0.5 to 0.8 W/cm² for electricdischarge. Thus, the silicon oxide film can be formed by discharge. Thesilicon oxide film manufactured in this way can then obtain preferablecharacteristics as the gate insulating film by thermal annealing at from400 to 500° C.

A first conductive film 5008 and a second conductive film 5009 forforming a gate electrode are formed on the gate insulating film 5007. Inthis embodiment, the first conductive film 5008 having a thickness offrom 50 to 100 nm is formed from Ta, and the second conductive film 5009having a thickness of from 100 to 300 nm is formed from W.

The Ta film is formed by a sputtering method, and the target of Ta issputtered by Ar. In this case, when suitable amounts of Xe and Kr areadded to Ar, internal stress of the Ta film is released, and pealing offthis film can be prevented. Resistivity of the Ta film of α phase isabout 20 μΩcm, and this Ta film can be used for the gate electrode.However, resistivity of the Ta film of β phase is about 180 μΩcm, and isnot suitable for the gate electrode. When tantalum nitride having acrystal structure close to that of the α phase of Ta and having athickness of about 10 to 50 nm is formed in advance as the base for theTa film to form the Ta film of the α phase, the Ta film of α phase canbe easily obtained.

The W film is formed by the sputtering method with W as a target.Further, the W film can be also formed by a thermal CVD method usingtungsten hexafluoride (WF₆). In any case, it is necessary to reduceresistance to use this film as the gate electrode. It is desirable toset resistivity of the W film to be equal to or smaller than 20 μΩcm.When crystal grains of the W film are increased in size, resistivity ofthe W film can be reduced. However, when there are many impurityelements such as oxygen, etc. within the W film, crystallization isprevented and resistivity is increased. Accordingly, in the case of thesputtering method, a W-target of 99.9999% or 99.99% in purity is used,and the W film is formed by taking a sufficient care of not mixingimpurities from a gaseous phase into the W film when the film is to beformed. Thus, a resistivity of from 9 to 20 μΩcm can be realized.

In this embodiment, the first conductive film 5008 is formed from Ta,and the second conductive film 5009 is formed from W. However, thepresent invention is not limited to this case. Each of these conductivefilms may also be formed from an element selected from Ta, W, Ti, Mo, Aland Cu, or an alloy material or a compound material having theseelements as principal components. Further, a semiconductor filmrepresented by a polysilicon film doped with an impurity element such asphosphorus may also be used. Examples of combinations other than thoseshown in this embodiment include: a combination in which the firstconductive film 5008 is formed from tantalum nitride (TaN), and thesecond conductive film 5009 is formed from W; a combination in which thefirst conductive film 5008 is formed from tantalum nitride (TaN), andthe second conductive film 5009 is formed from Al; and a combination inwhich the first conductive film 5008 is formed from tantalum nitride(TaN), and the second conductive film 5009 is formed from Cu.

Next, a mask 5010 is formed from a resist, and first etching processingfor forming an electrode and wiring is performed. In this embodiment, anICP (Inductively Coupled Plasma) etching method is used, and CF₄ and Cl₂are mixed with a gas for etching. RF (13.56 MHz) power of 500 W isapplied to the electrode of coil type at a pressure of 1 Pa so thatplasma is generated. RF (13.56 MHz) of 100 W power is also applied to asubstrate side (sample stage), and a substantially negative self biasvoltage is applied. When CF₄ and Cl₂ are mixed, the W film and the Tafilm are etched to the same extent.

Under the etching condition, end portions of a first conductive layerand a second conductive layer are formed into a tapered shape by effectsof the bias voltage applied to the substrate side by making the shape ofthe mask made of resist into an appropriate shape. The angle of a taperportion is set to from 15° to 45°. It is preferable to increase anetching time by a ratio of about 10 to 20% so as to perform the etchingwithout leaving the residue on the gate insulating film. Since aselection ratio of a silicon oxynitride film to the W film ranges from 2to 4 (typically 3), an exposed face of the silicon oxynitride film isetched by about 20 to 50 nm by over-etching processing. Thus, conductivelayers 5013 to 5014 of a first shape (first conductive layers 5013 a to5014 a and second conductive layers 5013 b to 5014 b) formed of thefirst and second conductive layers are formed by the first etchingprocessing. A region that is not covered with the conductive layers 5013to 5014 of the first shape is etched by about 20 to 50 nm in the gateinsulating film 5007, so that a thinned region is formed.

Then, an impurity element for giving an n-type conductivity is added byperforming first doping processing. A doping method may be either an iondoping method or an ion implantation method. The ion doping method iscarried out under the condition that a dose is set to from 1×10¹³ to5×10¹⁴ atoms/cm², and an acceleration voltage is set to from 60 to 100keV. An element belonging to group 15, typically, phosphorus (P) orarsenic (As) is used as the impurity element for giving the n-typeconductivity. However, phosphorus (P) is used here. In this case, theconductive layers 5013 to 5014 serve as masks with respect to theimpurity element for giving the n-type conductivity, and first impurityregions 5017 to 5018 are formed in a self-aligning manner. The impurityelement for giving the n-type conductivity is added to the firstimpurity regions 5017 to 5018 in a concentration range from 1×10²⁰ to1×10²¹ atoms/cm³ (FIG. 11B).

Second etching processing is next performed without removing the resistmask as shown in FIG. 11C. A W film is etched selectively by using CF₄,Cl₂ and O₂ as the etching gas. The conductive layers 5028 to 5029 of asecond shape (first conductive layers 5028 a to 5029 a and secondconductive layers 5028 b to 5029 b) are formed by the second etchingprocessing. A region of the gate insulating film 5007, which is notcovered with the conductive layers 5028 to 5029 of the second shape, isfurther etched by about 20 to 50 nm so that a thinned region is formed.

An etching reaction in the etching of the W film or the Ta film usingthe mixed gas of CF₄ and Cl₂ can be assumed from the vapor pressure of aradical or ion species generated and a reaction product. When the vaporpressures of a fluoride and a chloride of W and Ta are compared, thevapor pressure of WF₆ as a fluoride of W is extremely high, and vaporpressures of other WCl₅, TaF₅ and TaCl₅ are approximately equal to eachother. Accordingly, both the W film and the Ta film are etched using themixed gas of CF₄ and Cl₂. However, when a suitable amount of O₂ is addedto this mixed gas, CF₄ and O₂ react and become CO and F so that a largeamount of F-radicals or F-ions is generated. As a result, the etchingspeed of the W film whose fluoride has a high vapor pressure isincreased. In contrast to this, the increase in etching speed isrelatively small for the Ta film when F is increased. Since Ta is easilyoxidized in comparison with W, the surface of the Ta film is oxidized byadding O₂. Since no oxide of Ta reacts with fluorine or chloride, theetching speed of the Ta film is further reduced. Accordingly, it ispossible to make a difference in etching speed between the W film andthe Ta film so that the etching speed of the W film can be set to behigher than that of the Ta film.

As shown in FIG. 11D, a second doping processing is then performed. Inthis case, an impurity element for giving the n-type conductivity isdoped in a smaller dose than in the first doping processing and at ahigh acceleration voltage by reducing a dose lower than that in thefirst doping processing. For example, the acceleration voltage is set tofrom 70 to 120 keV, and the dose is set to 1×10¹³ atoms/cm². Thus, a newimpurity region is formed inside the first impurity region formed in theisland-like semiconductor layer in FIG. 11B. In the doping, theconductive layers 5028 to 5029 of the second shape are used as maskswith respect to the impurity element, and the doping is performed suchthat the impurity element is also added to regions underside the firstconductive layers 5028 a to 5029 a. Thus, third impurity regions 5034 to5035 are formed. The third impurity regions 5034 to 5035 containphosphorus (P) with a gentle concentration gradient that conforms withthe thickness gradient in the tapered portions of the first conductivelayers 5028 a to 5029 a. In the semiconductor layers that overlap thetapered portions of the first conductive layers 5028 a to 5029 a, theimpurity concentration is slightly lower around the center than at theedges of the tapered portions of the first conductive layers 5028 a to5029 a. However, the difference is very slight and almost the sameimpurity concentration is kept throughout the semiconductor layers.

A third etching treatment is then carried out as shown in FIG. 12A. CHF₆is used as etching gas, and reactive ion etching (RIE) is employed.Through the third etching treatment, the tapered portions of the firstconductive layers 5028 a to 5029 a are partially etched to reduce theregions where the first conductive layers overlap the semiconductorlayers. Thus formed are third shape conductive layers 5039 to 5040(first conductive layers 5039 a to 5040 a and second conductive layers5039 b to 5040 b). At this point, regions of the gate insulating film5007 that are not covered with the third shape conductive layers 5039 to5040 are further etched and thinned by about 20 to 50 nm.

Third impurity regions 5034 to 5035 are formed through the third etchingtreatment. The third impurity regions 5034 a to 5035 a that overlap thefirst conductive layers 5039 a to 5040 a, respectively, and secondimpurity regions 5034 b to 5035 b each formed between a first impurityregion and a third impurity region.

As shown in FIG. 12B, fourth impurity regions 5049 to 5054 having theopposite conductivity type to the first conductivity type are formed inthe island-like semiconductor layer 5005 for forming p-channel typeTFTs. The third shape conductive layer 5040 b is used as a mask againstthe impurity element and impurity regions are formed in a self-aligningmanner. At this point, the island-like semiconductor layer 5006 forforming n-channel type TFTs is entirely covered with a resist mask 5200.The impurity regions 5049 to 5054 have already been doped withphosphorus in different concentrations. The impurity regions 5049 to5054 are doped with diborane (B₂H₆) through ion doping and its impurityconcentrations are set to form 2×10²⁰ to 2×10²¹ atoms/cm³ in therespective impurity regions.

Through the steps above, the impurity regions are formed in therespective island-like semiconductor layers. The third shape conductivelayers 5039 to 5040 overlapping the island-like semiconductor layersfunction as gate electrodes.

After resist mask 5200 is removed, a step of activating the impurityelements added to the island-like semiconductor layers is performed tocontrol the conductivity type. This process is performed by a thermalannealing method using a furnace for furnace annealing. Further, a laserannealing method or a rapid thermal annealing method (RTA method) can beapplied. In the thermal annealing method, this process is performed at atemperature of from 400 to 700° C., typically from 500 to 600° C. withina nitrogen atmosphere in which oxygen concentration is equal to orsmaller than 1 ppm and is preferably equal to or smaller than 0.1 ppm.In this embodiment, heat treatment is performed for four hours at atemperature of 500° C. When a wiring material used in the third shapeconductive layers 5039 to 5040 is weak against heat, it is preferable toperform activation after an interlayer insulating film (having siliconas a principal component) is formed in order to protect wiring, etc.

When the activation is preformed by using the laser annealing method,the laser used in the crystallization can be used. When activation isperformed, the moving speed is set as well as the crystallizationprocessing, and the energy density of about 0.01 to 100 MW/cm²(preferably 0.01 to 10 MW/cm²) is required.

Further, the heat treatment is performed for 1 to 12 hours at atemperature of from 300 to 450° C. within an atmosphere including 3 to100% of hydrogen so that the island-like semiconductor layer ishydrogenerated. This step is to terminate a dangling bond of thesemiconductor layer by hydrogen thermally excited. Plasma hydrogenation(using hydrogen excited by plasma) may also be performed as anothermeasure for hydrogenation.

Next, as shown in FIG. 12C, a first interlayer insulating film 5055 isformed from a silicon oxynitride film with a thickness of 100 to 200 nm.The second interlayer insulating film 5056 from an organic insulatingmaterial is formed on the first interlayer insulating film. Thereafter,contact holes are formed through the first interlayer insulating film5055, the second interlayer insulating film 5056 and the gate insulatingfilm 5007. Each wirings 5059 to 5062 are patterned and formed.Thereafter, a pixel electrode 5064 coming in contact with the connectingwiring 5062 is patterned and formed.

A film having an organic resin as a material is used as the secondinterlayer insulating film 5056. Polyimide, polyamide, acrylic, BCB(benzocyclobutene), etc. can be used as this organic resin. Inparticular, since the second interlayer insulating film 5056 is providedmainly for planarization, acrylic excellent in leveling the film ispreferable. In this embodiment, an acrylic film having a thickness thatcan sufficiently level a level difference caused by the TFT is formed.The film thickness thereof is preferably set to from 1 to 5 μm (isfurther preferably set to from 2 to 4 μm).

In the formation of the contact holes, contact holes reaching n-typeimpurity region 5017 or p-type impurity regions 5049 and 5054 are formedrespectively by using dry etching or wet etching.

Further, a laminate film of a three-layer structure is patterned in adesired shape and is used as wirings (including a connecting wiring andsignal line) 5059 to 5062. In this three-layer structure, a Ti film witha thickness of 100 nm, an aluminum film containing Ti with a thicknessof 300 nm, and a Ti film with a thickness of 150 nm are continuouslyformed by the sputtering method. Of course, another conductive film mayalso be used.

In this embodiment, an ITO film of 110 nm in thickness is formed as apixel electrode 5064, and is patterned. Contact is made by arranging thepixel electrode 5064 such that this pixel electrode 5064 comes incontact with the connecting electrode 5062 and is overlapped with thisconnecting wiring 5062. Further, a transparent conductive film providedby mixing 2 to 20% of zinc oxide (ZnO) with indium oxide may also beused. This pixel electrode 5064 becomes an anode of the light emittingelement (FIG. 12A).

As shown in FIG. 12D, an insulating film (a silicon oxide film in thisembodiment) containing silicon wiht a thickness of 500 nm is nextformed. A third interlayer insulating film 5065 functions as a bank isformed in which an opening is formed in a position corresponding to thepixel electrode 5064. When the opening is formed, a side wall of theopening can easily be tapered by using the wet etching method. When theside wall of the opening is not gentle enough, deterioration of anorganic light emitting layer caused by a level difference becomes anotable problem.

Next, an organic light emitting layer 5066 and a cathode (MgAgelectrode) 5067 are continuously formed by using the vacuum evaporationmethod without exposing to the atmosphere. The organic light emittinglayer 5066 has a thickness of from 80 to 200 nm (typically from 100 to120 nm), and the cathode 5067 has a thickness of from 180 to 300 nm(typically from 200 to 250 nm).

In this process, the organic light emitting layer is sequentially formedwith respect to a pixel corresponding to red, a pixel corresponding togreen and a pixel corresponding to blue. In this case, since the organiclight emitting layer has an insufficient resistance against a solution,the organic light emitting layer must be formed separately for eachcolor instead of using a photolithography technique. Therefore, it ispreferable to cover a portion except for desired pixels using a metalmask so that the organic light emitting layer is formed selectively onlyin a required portion.

Namely, a mask for covering all portions except for the pixelcorresponding to red is first set, and the organic light emitting layerfor emitting red light are selectively formed by using this mask. Next,a mask for covering all portions except for the pixel corresponding togreen is set, and the organic light emitting layer for emitting greenlight are selectively formed by using this mask. Next, a mask forcovering all portions except for the pixel corresponding to blue issimilarly set, and the organic light emitting layer for emitting bluelight are selectively formed by using this mask. Here, different masksare used, but instead the same single mask may be used repeatedly.

Here, a system for forming three kinds of light emitting elementscorresponding to RGB is used. However, a system in which an lightemitting element for emitting white light and a color filter arecombined, a system in which the light emitting element for emitting blueor blue green light is combined with a fluorescent substance (afluorescent color converting medium: CCM), a system for overlapping thelight emitting elements respectively corresponding to R, G, and B withthe cathodes (opposite electrodes) by utilizing a transparent electrode,etc. may be used.

A known material can be used as the organic light emitting layer 5066.An organic material is preferably used as the known material inconsideration of a driving voltage. For example, a four-layer structureconsisting of a hole injection layer, a hole transportation layer, alight emitting layer and an electron injection layer is preferably usedfor the organic light emitting layer.

Next, the cathode 5067 is formed by using metal mask. This embodimentuses MgAg for the cathode 5067 but it is not limited thereto. Otherknown materials may be used for the cathode 5067.

Finally, a passivation film 5068 formed of silicon nitride film andhaving a thickness of 300 nm is formed. By forming the passivation film5068, the passivation film 5068 plays a role of protecting the organiclight emitting layer 5066 from moisture or the like. Thus, reliabilityof the light emitting element can be further improved.

Accordingly, the light emitting device having a structure shown in FIG.12D is completed.

The light emitting dievice in this embodiment has very high reliabilityand improved operation characteristics by arranging the TFTs of theoptimal structure in a driving circuit portion in addition to the pixelportion. Further, in a crystallization process, crystallinity can bealso improved by adding a metal catalyst such as Ni. Thus, a drivingfrequency of the signal line driving circuit can be set to 10 MHz ormore.

First, the TFT having a structure for reducing hot carrier injection soas not to reduce an operating speed as much as possible is used as ann-channel type TFT of a CMOS circuit forming the driving circuitportion. Here, the driving circuit includes a shift register, a buffer,a level shifter, a latch in line sequential driving, a transmission gatein dot sequential driving, etc.

In the case of this embodiment, an active layer of the n-channel typeTFT includes a source region, a drain region, an overlapping LDD region(L_(OV) region) that is overlapped with the gate electrode through thegate insulating film, an offset LDD region (L_(OFF) region) that is notoverlapped with the gate electrode through the gate insulating film, andchannel forming region.

Deterioration by the hot carrier injection in the p-channel type TFT ofthe CMOS circuit is almost negligible. Therefore, it is not necessary toparticularly form the LDD region in this p-channel type TFT. However,similar to the n-channel type TFT, the LDD region can be formed in thep-channel type TFT as a hot carrier countermeasure.

Further, when the CMOS circuit for bi-directionally flowing an electriccurrent through a channel forming region, i.e., the CMOS circuit inwhich roles of the source and drain regions are exchanged is used in thedriving circuit, it is preferable for the n-channel type TFT thatconstitutes the CMOS circuit to form LDD regions such that the channelforming region is sandwiched between the LDD regions. As an example ofthis, a transmission gate used in the dot sequential driving is given.When a CMOS circuit required to reduce an OFF-state current value asmuch as possible is used in the driving circuit, the n-channel type TFTforming the CMOS circuit preferably has a L_(OV) region. Thetransmission gate used in the dot sequential driving can be given alsoas an example as such.

In practice, the device reaching the state of FIG. 12D is packaged(enclosed) using a protective film that is highly airtight and allowslittle gas to transmit (such as a laminate film and a UV-curable resinfilm) or a light-transmissive sealing material, so as to further avoidexposure to the outside air. A space inside the seal may be set to aninert atmosphere or a hygroscopic substance (barium oxide, for example)may be placed there to improve the reliability of the light emittingelement.

After securing the airtightness through packaging or other processing, aconnector (flexible printed circuit: FPC) is attached for connecting anexternal signal terminal with a terminal led out from the elements orcircuits formed on the substrate. The device in a state that can beshipped is called display device in this specification.

Furthermore, in accordance with the processes shown in this embodiment,the number of photomasks can be reduced that is need for manufacturingthe light emitting device. As a result, the processes can be reduced,and this contributes to a reduction in the manufacturing costs and anincrease in throughput.

The method of manufacturing the light emitting device of the presentinvention is not limited to the method of manufacturing the lightemitting device described in this embodiment. Therefore, the lightemitting device of the present invention can be fabricated by knownmethod.

This embodiment can be executed by freely combining with configurationsshown in Embodiment Modes 1 to 2, and Embodiments 1 to 4.

Embodiment 6

In this embodiment, an external appearance of the light emitting deviceof the present invention is described with reference to FIGS. 13A to13C. In this embodiment, a light emitting element is referred to as anOLED element. However, other light emitting element may be used in placeof the OLED element.

FIG. 13A is a top view of the light emitting device which is formedaccording as the element substrate with the transistor is sealed bysealing materials, FIG. 13B is a cross sectional view taken along with aline A-A′ of FIG. 13A, and FIG. 13C is a cross sectional view takenalong with a line B-B′ of FIG. 13A.

A seal member 4009 is provided so as to surround a pixel portion 4002, asource signal line driving circuit 4003, and writing and initializinggate signal line driving circuits 4000 a, 4000 b, which are provided ona substrate 4001. Further, a sealing material 4008 is provided on thepixel portion 4002, the signal line driving circuit 4003, and thewriting and initializing gate signal line driving circuits 4004 a, 4004b. Thus, the pixel portion 4002, the source signal line driving circuit4003, and the writing and initializing gate signal line driving circuits4004 a, 4004 b are sealed by the substrate 4001, the seal member 4009and the sealing material 4008. Although reference numeral 4210 is ahollow portion, a filler can be implanted in the hollow portion.

Further, the pixel portion 4002, the source signal line driving circuit4003, and the writing and initializing gate signal line driving circuits4004 a, 4004 b, which are provided on the substrate 4001, have aplurality of TFTs. In FIG. 13B, a TFT, hereafter referred to as adriving TFT (hereafter, only an n-channel TFT and a p-channel TFT areshown in the figure) 4201 included in the source signal line drivingcircuit 4003 and a driving element 4202 included in the pixel portion4002, which are formed on a base film 4010, are typically shown.

In this embodiment, the p-channel TFT or the n-channel TFT manufacturedby a known method is used as the driving TFT 4201, and the n-channel TFTmanufactured by a known method is used as an initializing element 103(not shown in FIG. 13).

An interlayer insulating film (leveling film) 4301 is formed on thedriving TFT 4201 and the driving element 4202, and a pixel electrode(anode) 4203 electrically connected to a drain of the driving element4202 is formed thereon. A transparent conductive film having a largework function is used for the pixel electrode 4203. A compound of indiumoxide and tin oxide, a compound of indium oxide and zinc oxide, zincoxide, tin oxide or indium oxide can be used for the transparentconductive film. The transparent conductive film added with gallium mayalso be used.

Then, an insulating film 4302 is formed on the pixel electrode 4203, andthe insulating film 4302 is formed with an opening portion on the pixelelectrode 4203. In this opening portion, an organic light emitting layer4204 is formed on the pixel electrode 4203. A known organic lightemitting material or inorganic light emitting material may be used forthe organic light emitting layer 4204. Further, there exist a lowmolecular weight material and a polymeric material as the organic lightemitting materials, and both the materials may be used.

A known evaporation technique or application technique may be used as amethod of forming the organic light emitting layer 4204. Further, thestructure of the organic light emitting layer may take a laminationstructure by freely combining a hole injection layer, a holetransporting layer, a light emitting layer, electron transporting layer,and electron injection layer. Also, the structure of the organic lightemitting layer may take a single layer structure.

A cathode 4205 made of a conductive film having light-shielding property(typically, conductive film containing aluminum, copper or silver as itsmain constituent or lamination film of the conductive film and anotherconductive film) is formed on the organic light emitting layer 4204.Further, it is desirable that moisture and oxygen that exist on aninterface of the cathode 4205 and the organic light emitting layer 4204are removed as much as possible. Therefore, such a device is necessarythat the organic light emitting layer 4204 is formed in a nitrogen orrare gas atmosphere, and then, the cathode 4205 is formed withoutexposure to oxygen and moisture. In this embodiment, the described filmdeposition is enabled by using a multi-chamber type (cluster tool type)film forming device.

As described above, a light emitting element 4303 constituted of thepixel electrode (anode) 4203, the organic light emitting layer 4204 andthe cathode 4205 is formed. Further, a protective film 4209 is formed onthe insulating film 4302 so as to cover the light emitting element 4303.The protective film 4209 is effective in preventing the intrusion ofoxygen, moisture and the like from the light emitting element 4303.

Reference numeral 4005 a denotes a wiring drawn to be connected to thepower supply line, and the wiring 4005 a is electrically connected to asource region of the driving element 4202. The drawn wiring 4005 apasses between the seal member 4009 and the substrate 4001, and iselectrically connected to an FPC wiring 4301 of an FPC 4006 through ananisotropic conductive film 4300.

A glass material, a metal material (typically, stainless material), aceramics material or a plastic material (including a plastic film) canbe used for the sealing material 4008. As the plastic material, an FRP(fiberglass-reinforced plastics) plate, a PVF (polyvinyl fluoride) film,a Mylar film, a polyester film or an acrylic resin film may be used.Further, a sheet with a structure in which an aluminum foil issandwiched with the PVF film or the Mylar film can also be used.

However, in the case where the light from the light emitting element isemitted toward the cover member side, the cover member needs to betransparent. In this case, a transparent substance such as a glassplate, a plastic plate, a polyester film or an acrylic film is used.

Further, in addition to an inert gas such as nitrogen or argon, anultraviolet curable resin or a thermosetting resin may be used as thefiller 4103, so that PVC (polyvinyl chloride), acrylic, polyimide, epoxyresin, silicone resin, PVB (polyvinyl butyral) or EVA (ethylene vinylacetate) can be used. In this embodiment, nitrogen is used for thefiller.

Moreover, a concave portion 4007 is provided on the surface of thesealing material 4008 on the substrate 4001 side, and a hygroscopicsubstance or a substance that can absorb oxygen 4207 is arranged thereinin order that the filler 4103 is made to be exposed to the hygroscopicsubstance (preferably, barium oxide) or the substance that can absorboxygen. Then, the hygroscopic substance or the substance that can absorboxygen 4207 is held in the concave portion 4007 by a concave portioncover member 4208 such that the hygroscopic substance or the substancethat can absorb oxygen 4207 is not scattered. Note that the concaveportion cover member 4208 has a fine mesh form, and has a structure inwhich air and moisture are penetrated while the hygroscopic substance orthe substance that can absorb oxygen 4207 is not penetrated. Thedeterioration of the light emitting element 4303 can be suppressed byproviding the hygroscopic substance or the substance that can absorboxygen 4207.

As shown in FIG. 13C, the pixel electrode 4203 is formed, and at thesame time, a conductive film 4203 a is formed so as to contact with thesurface of the drawn wiring 4005 a.

Further, the anisotropic conductive film 4300 has conductive filler 4300a. The conductive film 4203 a on the substrate 4001 and the FPC wiring4301 on the FPC 4006 are electrically connected to each other by theconductive filler 4300 a by heat-pressing the substrate 4001 and the FPC4006.

Note that the structure of Embodiment 6 can be implemented by beingfreely combined with the structures shown in Embodiment Modes 1 to 2,and Embodiments 1 to 5.

Embodiment 7

Light emitting materials used in light emitting elements are classifiedroughly into a low molecular weight material and a polymeric material.The light emitting device of the present invention can employ a lowmolecular weight organic light emitting material and a polymeric organiclight emitting material both. Further, materials which are difficult tobe classified into a low molecular weight material and a polymericmaterial (such as a material disclosed in Japanese Patent ApplicationNo. 2001-167508) may be used according to circumstances.

A low molecular weight organic light emitting material is formed into afilm by evaporation. This makes it easy to form a laminate structure,and the efficiency is increased by layering films of different functionssuch as a hole transporting layer and an electron transporting layer. Ofcourse, a hole transporting layer, an electron transporting layer andthe like do not always exist clearly, and a single layer or a pluralityof layers of the hole transporting layer and the electron transportinglayer in mixed state may be exist (for example, Japanese PatentApplication No. 2001-020817, etc. are referred to), thereby elongationthe lifetime of the element and improvement of the light emittingefficiency of the element may be achieved.

Examples of low molecular weight light emitting material include analuminum complex having quinolinol as a ligand (Alq₃), a triphenylaminederivative (TPD) and the like.

On the other hand, a polymeric light emitting material is physicallystronger than a low molecular weight light emitting material andenhances the durability of the element. Furthermore, a polymericmaterial can be formed into a film by application and thereforemanufacture of the element is relatively easy.

The structure of a light emitting element using a polymeric lightemitting material is basically the same as the structure of a lightemitting element using a low molecular weight light emitting material,and has a cathode, an organic light emitting layer, and an anode. Whenan organic light emitting layer is formed from a polymeric organic lightemitting material, a two-layer structure is popular among the knownones. This is because it is difficult to form a laminate structure usinga polymeric material unlike the case of using a low molecular weightorganic light emitting material. Specifically, an element using apolymeric organic light emitting material has a cathode, a lightemitting layer, a hole transporting layer, and an anode. Note that, Camay be employed as the cathode material in a light emitting elementusing a polymeric light emitting material.

The color of light emitted from an element is determined by the materialof its light emitting layer. Therefore, a light emitting element thatemits light of desired color can be formed by choosing an appropriatematerial. The polymeric organic light emitting material that can be usedto form a light emitting layer is a polyparaphenylene vinylene-basedmaterial, a polyparaphenylene-based material, a polythiophen-basedmaterial, or a polyfluorene-based material.

The polyparaphenylene vinylene-based material is a derivative ofpoly(paraphenylene vinylene) (denoted by PPV), for example, poly(2,5-dialkoxy-1, 4-phenylene vinylene) (denoted by RO-PPV),poly(2-(2′-ethyl-hexoxy)-5-metoxy-1, 4-phenylene vinylene) (denoted byMEH-PPV), poly(2-(dialkoxyphenyl)-1, 4-phenylene vinylene) (denoted byROPh-PPV), etc.

The polyparaphenylene-based material is a derivative ofpolyparaphenylene (denoted by PPP), for example,poly(2,5-dialkoxy-1,4-phenylene) (denoted by RO-PPP),poly(2,5-dihexoxy-1,4-phenylene), etc.

The polythiophene-based material is a derivative of polythiophene(denoted by PT), for example, poly(3-alkylthiophene) (denoted by PAT),poly(3-hexylthiophene) (denoted by PHT), poly(3-cyclohexylthiophene)(denoted by PCHT), poly(3-cyclohexyl-4-methylthiophene) (denoted byPCHMT), poly(3, 4-dicyclohexylthiophene) (denoted by PDCHT),poly[3-(4-octylphenyl)-thiophene] (denoted by POPT), andpoly[3-(4-octylphenyl)-2, 2 bithiophene] (denoted by PTOPT), etc.

The polyfluorene-based material is a derivative of polyfluorene (denotedby PF), for example, poly(9, 9-dialkylfluorene) (denoted by PDAF) andpoly(9, 9-dioctylfluorene) (denoted by PDOF), etc.

If a layer that is formed of a polymeric light emitting material capableof transporting holes is sandwiched between an anode and a polymericlight emitting material layer that emits light, injection of holes fromthe anode is improved. This hole transporting material is generallydissolved into water together with an acceptor material, and thesolution is applied by spin coating or the like. Since the holetransporting material is insoluble in an organic solvent, the filmthereof can form a laminate with the above-mentioned light emittingmaterial layer that emits light.

A mixture of PEDOT and camphor sulfonic acid (denoted by CSA) thatserves as the acceptor material, a mixture of polyaniline (denoted byPANI) and polystyrene sulfonic acid (denoted by PSS) that serves as theacceptor material and the like can be given as the polymeric lightemitting material capable of transporting holes.

The structure of this embodiment may be freely combined with any of thestructures of Embodiment Modes 1 to 2, and Embodiments 1 to 6.

Embodiment 8

The light emitting device can be used for a display device of variousequipments by utilizing the characteristics of the light emittingelement. For example, the light emitting device using an OLED as a lightemitting element exhibits more excellent recognizability of the displayimage because of the high contrast of brightness and darkness and thelight emitting device has a wider viewing angle as compared to a liquidcrystal display device. Therefore, the light emitting device isadvantageous for the monitoring usage. Furthermore, the light emittingdevice is highly advantageous to a display device for displayinganimation since the light emitting device has a characteristic of highspeed response. Also, the light emitting device is advantageous to aportable apparatus since the light emitting device is thin and lightweight.

Such electronic devices using a light emitting device of the presentinvention include a video camera, a digital camera, a goggles-typedisplay (head mount display), a navigation system, a sound reproductiondevice (such as a car audio equipment and an audio set), a lap-topcomputer, a game machine, a portable information terminal (such as amobile computer, a mobile telephone, a portable game machine, and anelectronic book), an image reproduction apparatus including a recordingmedium (more specifically, an apparatus which can reproduce a recordingmedium such as a digital versatile disc (DVD) and so forth, and includesa display for displaying the reproduced image), or the like. Inparticular, in the case of the portable information terminal, use of thelight emitting device is preferable, since the portable informationterminal that is likely to be viewed from a tilted direction is oftenrequired to have a wide viewing angle. FIGS. 14A to 14H respectivelyshows various specific examples of such electronic devices.

FIG. 14A illustrates a display device which includes a casing 2001, asupport table 2002, a display portion 2003, a speaker portion 2004, avideo input terminal 2005 and the like. The present invention isapplicable to the display portion 2003. The light emitting device is ofthe self-emission-type and therefore requires no backlight. Thus, thedisplay portion thereof can have a thickness thinner than that of theliquid crystal display device. The light emitting element display deviceis including the entire display device for displaying information, suchas a personal computer, a receiver of TV broadcasting and an advertisingdisplay.

FIG. 14B illustrated a digital still camera which includes a main body2101, a display portion 2102, an image receiving portion 2103, anoperation key 2104, an external connection port 2105, a shutter 2106,and the like. The light emitting device of the present invention can beused as the display portion 2102.

FIG. 14C illustrates a lap-top computer which includes a main body 2201,a casing 2202, a display portion 2203, a keyboard 2204, an externalconnection port 2205, a pointing mouse 2206, and the like. The lightemitting device of the present invention can be used as the displayportion 2203.

FIG. 14D illustrated a mobile computer which includes a main body 2301,a display portion 2302, a switch 2303, an operation key 2304, aninfrared port 2305, and the like. The light emitting device of thepresent invention can be used as the display portion 2302.

FIG. 14E illustrates a portable image reproduction apparatus including arecording medium (more specifically, a DVD reproduction apparatus),which includes a main body 2401, a casing 2402, a display portion A2403, another display portion B 2404, a recording medium (DVD or thelike) reading portion 2405, an operation key 2406, a speaker portion2407 and the like. The display portion A 2403 is used mainly fordisplaying image information, while the display portion B 2404 is usedmainly for displaying character information. The light emitting deviceof the present invention can be used as these display portions A 2403and B 2404. The image reproduction apparatus including a recordingmedium further includes a game machine or the like.

FIG. 14F illustrates a goggle type display (head mounted display) whichincludes a main body 2501, a display portion 2502, arm portion 2503, andthe like. The light emitting device of the present invention can be usedas the display portion 2502.

FIG. 14G illustrates a video camera which includes a main body 2601, adisplay portion 2602, a casing 2603, an external connecting port 2604, aremote control receiving portion 2605, an image receiving portion 2606,a battery 2607, a sound input portion 2608, an operation key 2609, andthe like. The light emitting device of the present invention can be usedas the display portion 2602.

FIG. 14H illustrates a mobile telephone which includes a main body 2701,a casing 2702, a display portion 2703, a sound input portion 2704, asound output portion 2705, an operation key 2706, an external connectingport 2707, an antenna 2708, and the like. The light emitting device ofthe present invention can be used as the display portion 2703. Note thatthe display portion 2703 can reduce power consumption of the mobiletelephone by displaying white-colored characters on a black-coloredbackground.

When the brighter luminance of light emitted from the light emittingmaterial becomes available in the future, the light emitting device ofthe present invention will be applicable to a front-type or rear-typeprojector in which light including output image information is enlargedby means of lenses or the like to be projected.

The aforementioned electronic devices are more likely to be used fordisplay information distributed through a telecommunication path such asInternet, a CATV (cable television system), and in particular likely todisplay moving picture information. Therefore, the light emitting deviceusing the light emitting element of first response speed is highlyadvantageous.

A portion of the light emitting device that is emitting light consumespower, so it is desirable to display information in such a manner thatthe light-emitting portion therein becomes as small as possible.Accordingly, when the light emitting device is applied to a displayportion which mainly displays character information, e.g., a displayportion of a portable information terminal, and more particular, aportable telephone or a sound reproduction device, it is desirable todrive the light emitting device so that the character information isformed by a light emitting portion while a non-emission portioncorresponds to the background.

As set forth above, the present invention can be applied variously to awide range of electronic devices in all fields. The electronic device inthis embodiment can be obtained by utilizing a light emitting devicehaving the configuration in which the structures in Embodiments 1through 8 are freely combined.

Using a multi-drain transistor, which is a semiconductor elementaccording to the invention, enables to form a circuit, which isdifficult to be constructed only with a conventional single draintransistor. Otherwise, a circuit, which can be constructed only with aconventional single drain transistor but becomes complicated or requireslarge area, can be provided without such disadvantage by using amulti-drain transistor.

The current storing circuit according to the invention comprises awriting element and a driving element in either or both of which amulti-drain transistor is used. Therefore, it is useful in simplifying,reducing in the area and highly integrating various circuits requiring acurrent storing function such as a current signal buffer. Further, thehigh yield in manufacturing can be expected since the number of devicesis small.

In accordance with the light-emitting device in the invention, it ispossible by current-driving the luminous element to maintain the currentflowing to the luminous element existing in a display device well evenin the following cases: the case that the electric resistance of theluminous element depends on ambient temperature; and the case that thevoltage-driving of the luminous element lowers the luminous intensity asthe time elapses. Maintaining the current flowing to the luminouselement well enables the luminous intensity to be kept good. As aresult, color shift can be also avoided in a color display device of thetype that respective sub-pixels in RGB are separately formed.

Current-driving of the luminous element enables significant differencebetween pixels in the amount of the current flowing to the luminouselement to be prevented from occurring even when a characteristic of thedriving element for controlling the current flowing to the luminouselement is different between pixels, so that the uneven intensity of adisplay screen can be also restrained.

Furthermore, the current flowing to the luminous element can be kept atthe desired value, so that change of gradation due to potential fallcaused by wiring resistance can be prevented. This is also an advantagein comparison with the voltage-driving of the luminous element.

Moreover, in accordance with the light-emitting device of the invention,using a semiconductor element, that is, a multi-drain transistoraccording to the invention in a pixel circuit enables the area occupiedby the pixel circuit to be reduced. As a result, the aperture raterises, and thereby, the density of the current flowing to the luminouselement decreases, so that saving the electric power and stoppingdeterioration of the luminous element per se can be performed.

In the display device according to the invention, a pixel circuit canalso be reduced in the area, highly integrated and highly functionalizedsince a multi-drain transistor, which is a semiconductor elementaccording to the invention, is used in the pixel circuit.

The electronic devices according to the invention have advantages ofhigh functionality and high reliability since the light-emitting deviceor a display device according to the invention, which has thecharacteristics described above, is mounted thereto.

1. A light-emitting device comprising a pixel, the pixel comprising aluminous element, a writing element, a driving element, an initializingelement, and a capacitance element, wherein the writing elementcomprises a first semiconductor layer, a first gate electrode, a firstgate insulating film between the first semiconductor layer and the firstgate electrode, the first semiconductor layer comprising a first channelforming region, a first impurity region, a second impurity region and athird impurity region; wherein the driving element comprises a secondsemiconductor layer, a second gate electrode, a second gate insulatingfilm between the second semiconductor layer and the second gateelectrode, the second semiconductor layer comprising a second channelforming region, a fourth impurity region, a fifth impurity region, and asixth impurity region; wherein the first impurity region is connected toa source signal line, the second impurity region is connected to one ofsource and drain regions of the initializing element, and the thirdimpurity is connected to the sixth impurity region of the drivingelement in the writing element, wherein the other of the source regionand the drain region of the initializing element is connected to a powersupply line, and wherein the fourth impurity region is connected to thepower supply line, the fifth impurity region is connected to a pixelelectrode of the luminous element.
 2. A light-emitting device accordingto claim 1, wherein an channel width of a part of the second channelforming region attached to the fifth impurity region is longer thanchannel widths of parts of the second channel forming region attached tothe fourth and the sixth impurity regions.
 3. A light-emitting deviceaccording to claim 1, wherein the light-emitting device is selected fromthe group consisting of an OLED display device, a digital still camera,a lap-top computer, a mobile computer, a portable image reproductionapparatus, a goggle type display, a video camera, and a mobiletelephone.
 4. An electronic device having the light-emitting deviceaccording to claim 1, wherein the electronic device is selected from thegroup consisting of an OLED display device, a digital still camera, alap-top computer, a mobile computer, a portable image reproductionapparatus, a goggle type display, a video camera, and a mobiletelephone.